CREATE TYPE platforms AS ENUM ('p1', 'p2', 'p3');

CREATE TABLE mytable (
    id bigint NOT NULL DEFAULT nextval('mytable_sq'::regclass),
    related_id integer NOT NULL,
    platform platforms NOT NULL DEFAULT 'default'::platforms,
    name character varying(200) NOT NULL,
    CONSTRAINT mytable_pkey PRIMARY KEY (id),
    CONSTRAINT mytable_related_id_fkey FOREIGN KEY (related_id)
         REFERENCES related (id)
);

CREATE INDEX related_id__platform__index ON mytable (related_id, platform);
CREATE UNIQUE INDEX some_other_index ON mytable (related_id, lower(name::text));

EXPLAIN ANALYZE
SELECT * FROM mytable
WHERE related_id=1 AND platform='p2'::platforms
LIMIT 20;

 Limit  (cost=0.00..14.07 rows=20 width=737) (actual time=12863.465..12863.465 rows=0 loops=1)
    ->  Seq Scan on mytable  (cost=0.00..1492790.47 rows=2122653 width=737) (actual time=12863.464..12863.464 rows=0 loops=1)
          Filter: ((related_id = 1) AND (platform = 'p2'::platforms))
          Rows Removed by Filter: 21076656
 Planning time: 3.540 ms
 Execution time: 12868.190 ms

EXPLAIN ANALYZE
SELECT * FROM mytable
WHERE related_id=1 AND platform='p2'::platforms;

 Bitmap Heap Scan on mytable  (cost=60533.63..1295799.94 rows=2122653 width=737) (actual time=0.890..0.890 rows=0 loops=1)
 Recheck Cond: ((related_id = 1) AND (platform = 'p2'::platforms))
  ->  Bitmap Index Scan on related_id__platform__index  (cost=0.00..60002.97 rows=2122653 width=0) (actual time=0.888..0.888 rows=0 loops=1)
         Index Cond: ((related_id = 1) AND (platform = 'p2'::platforms))
 Planning time: 0.827 ms
 Execution time: 1.104 ms

EXPLAIN ANALYZE
SELECT * FROM mytable
WHERE related_id=1 AND platform='p1'::platforms
LIMIT 20;

 Limit  (cost=0.44..70.95 rows=20 width=737) (actual time=0.759..0.995 rows=20 loops=1)
   ->  Index Scan using related_id__platform__index on mytable  (cost=0.44..1217669.26 rows=345388 width=737) (actual time=0.759..0.993 rows=20 loops=1)
         Index Cond: ((related_id = 1) AND (platform = 'p1'::platforms))
 Planning time: 5.776 ms
 Execution time: 2.476 ms

EXPLAIN ANALYZE
SELECT * FROM mytable
WHERE related_id=1 AND platform='p3'::platforms LIMIT 20;

 Limit  (cost=0.44..80.37 rows=20 width=737) (actual time=0.014..0.016 rows=2 loops=1)
   ->  Index Scan using related_id__platform__index on mytable  (cost=0.44..99497.62 rows=24894 width=737) (actual time=0.014..0.015 rows=2 loops=1)
         Index Cond: ((related_id = 1) AND (platform = 'p3'::platforms))
 Planning time: 0.972 ms
 Execution time: 0.123 ms

SELECT * FROM tbl
WHERE  related_id = 1 AND platform = 'p2'::platforms
LIMIT  20;

CREATE INDEX related_id_1_platform_2_idx ON tbl (id)
WHERE  related_id = 1 AND platform = 'p2'::platforms;

CREATE INDEX related_id_1_idx ON tbl (platform)
WHERE  related_id = 1;

CREATE INDEX related_id_1_func_idx ON tbl ((platform::text::platforms))  -- double parens!
WHERE  related_id = 1;

SELECT *
FROM   pg_stats
WHERE  schemaname = 'public'  -- actual schema
AND    tablename  = 'related_id_1_func_idx';  -- actual idx name

SELECT ...
WHERE related_id = 1 AND platform::text::platforms = 'p2'::platforms;